1. Field of the Invention
The present invention relates to a holey fiber.
2. Description of the Related Art
A holey fiber or a photonic crystal fiber is a new type of optical fiber that has a core region formed at a center of the optical fiber and a cladding region surrounding the core region and having a plurality of holes around the core region, where the cladding region has the reduced average refractive index because of the presence of the holes so that a light propagates through the core region by the principle of the total reflection of light. Because the refractive index is controlled by the holes, the holey fibers can realize unique properties such as endlessly single mode (ESM) characteristics and a zero-dispersion wavelength shifted towards extremely shorter wavelengths, which cannot be realized with conventional optical fibers. The ESM means that a cut-off wavelength is not present and a light is transmitted in a single mode at all wavelengths. With the ESM, it is possible to realize an optical transmission at a high transmission speed over a broad bandwidth.
In recent years, a technology related to ytterbium-doped optical fibers (YDF) is maturing, which can be used as amplifying optical fibers at a 1.0-micrometer wavelength band (e.g., 1000 nanometers to 1100 nanometers) around a wavelength of 1050 nanometers. The demand for optical fibers that can be applied to fiber lasers for the 1.0-micrometer wavelength band, optical fibers for supercontinuum (SC) light sources, or optical transmission lines has thus increased. Holey fibers are being hoped to meet this demand. For example, experimental results of performing optical transmission in a wide band including a wavelength of 1064 nanometers using a holey fiber as an optical transmission line have been reported (see K. Ieda, et al., “Visible to Infrared WDM transmission over PCF”, ECOC 2006-Tu3.3.4, 2006).
However, in the conventional holey fiber, a wavelength dispersion value in the wavelength band of 1.0 μm is about −20 ps/nm/km or smaller. That is, its absolute value is relatively large. Therefore, when the conventional holey fiber is used as an optical transmission line for a long-haul transmission of optical signals in the wavelength band of 1.0 μm, the optical signals are extremely distorted, which is problematic.